As disclosed in Japanese Laid-Open Patent Publication No. 2003-284057 (hereinafter referred to as “JP2003-284057A”), it has heretofore been customary for a vehicle periphery monitoring apparatus to detect an object such as a pedestrian or the like that has a possibility of colliding with a vehicle, from images (a grayscale image and a binary image converted therefrom) of the periphery of the vehicle, which are captured by infrared cameras, and to provide information concerning the detected object to the driver of the vehicle.
More specifically, the vehicle periphery monitoring apparatus disclosed in JP2003-284057A detects, as an object, a high-temperature area in images of the periphery of the vehicle, which are captured by a set of left and right infrared cameras (stereo cameras), and calculates the distance up to the object by determining a parallax between the object in the left and right images. The vehicle periphery monitoring apparatus detects an object such as a pedestrian or the like that is likely to affect travelling of the vehicle, i.e., that has a possibility of colliding with the vehicle, based on the position of the object and the direction in which the object moves, and issues a warning concerning the detected object (see paragraphs [0014] and [0018] of JP2003-284057A).
However, since such a vehicle periphery monitoring apparatus including a pair of left and right infrared cameras is expensive, this type of vehicle periphery monitoring apparatus has been incorporated in limited luxury vehicles only.
In an attempt to reduce the cost of vehicle periphery monitoring apparatus, a vehicle periphery monitoring apparatus, as disclosed in Japanese Patent No. 4521642 (hereinafter referred to as “JP4521642B”) employs a single vehicle-mounted infrared camera, which captures at least two images (two frames) of an object in the vicinity of a vehicle within a given time interval. As the relative speed between the object and the vehicle that incorporates the vehicle periphery monitoring apparatus therein is higher, the size of an image of the object in a later-captured image changes more greatly than the size of an image of the object in an earlier-captured image. Since the relative speed between the object and the vehicle is higher, the object, which is present in front of the vehicle, reaches the vehicle in a shorter period of time. Consequently, even a single infrared camera is able to monitor the periphery of a vehicle by estimating a period of time that an object takes to reach the vehicle, i.e., a so-called TTC (Time To Contact or Time to Collision), from the rate of change of the size of images of an object, which are captured within a given time interval (see paragraphs [0019] and [0020] of JP4521642B).